LONDON, June 8 (Reuters) - Babies born just 1 or 2 weeks before their 40-week gestation due date are more likely to develop learning difficulties such as autism or dyslexia, according to a British study published on Tuesday.

The findings show that even babies born at 39 weeks -- the point at which many women choose to have a Caesarean section delivery -- have an increased risk of a developing a learning disability compared with babies born a week later at 40 weeks.

Scientists in Scotland, analysing the birth history of more than 400,000 schoolchildren, found that while babies born at 40 weeks have a 4 percent risk of learning difficulties, those born at 37 to 39 weeks of gestation have a 5.1 percent risk.

"There was an increasing risk of special educational needs as the gestation date fell, so as deliveries got earlier, the risk went up," said Jill Pell, an expert in public health and health policy Glasgow University, who led the study.

"Even being just a week early put the risk up."

It is already known that a baby born prematurely -- for example at 24 weeks of gestation -- is more likely to have learning difficulties. But the risks for babies born in the 24 to 40 week range had not previously been studied.

According to the World Health Organisation, more and more women worldwide are delivering by caesarean section and a "significant proportion" of these surgical procedures are performed without any clear medical need.

Around a third of babies are born between 37 and 39 weeks of gestation, either by caesarean section or natural vaginal delivery.

Rates of autism have also been rising, with worldwide prevalence estimated at between 1 to 2 people in every thousand, although experts say this may be partly due to better diagnosis.

Pell noted this rise but said it would be "a leap too far" to link her findings directly to rates of autism, since autism was only one of a range of learning difficulties considered.

Pell, whose study was published in the Public Library of Science Medicine journal, stressed that caesarean sections were not the only factor behind early-term births, since some women go into labour naturally before 40 weeks of gestation.

But she said doctors and women should consider the risks of learning difficulties when thinking about a caesarean.

"It is now normal policy (in caesarean section) to deliver women a week early," she said in a telephone interview. "But if you make a decision...for an elective pre-term delivery, then it has to be a balance, weighing up the risks and potential benefits.

"What this study shows is that special education needs are another factor that need to be considered."

Children with special educational needs may have either a learning difficulty such as dyslexia or autism, or a physical difficulty such as deafness or poor vision.

Pell found that although the risk of educational difficulties was much higher in preterm than in early term babies, the absolute numbers of children with difficulties in the 37 to 39 week group were higher, because many more babies are born at this time than before 37 weeks.

In her study, early term births accounted for 5.5 percent of cases learning disabilities, while preterm deliveries accounted for only 3.6 percent of cases.